Various methods have been proposed for synchronizing the clock frequency and time of a plurality of devices by using a burst signal having no synchronization information, such as a packet. There are standard specifications such as IETF Network Time Protocol (NTP), IEEE 1588-2002 Precision Time Protocol (PTP) version. 1, ITU-TG. 8282/Y. 1362 Synchronous Ethernet®, and Pseudo Wire Emulation Edge to Edge (PWE3).
When high-precision synchronization of the clock frequency and time is required, the communication standards, such as TDD LTE, which require a frequency precision in units of ppb and a time precision in units of μs, employ a method such as IEEE 1588-2008 PTP version. 2, or a method of synchronizing with Global Navigation Satellite Systems (GNSS) in each device.
In IEEE 1588 PTP, a master device transmits a timing packet to a slave device and the slave device performs processing for reproducing the frequency and time of the slave device based on a time stamp value included in the packet. At this time, the propagation delay itself of the timing packet and a fluctuation in the delay affect the synchronization accuracy.
In view of this, IEEE recommends a method of compensating for a delay in a slave device by adding a propagation delay due to a physical transmission rate between devices and a buffer retention period within devices to a specific field of a timing packet.
In a similar technique, Patent Literature 1 discloses a system in which a master station device and a slave station device perform time synchronization by using Gigabit Ethernet-Passive Optical Network (GE-PON), which is an asynchronous multiplex transmission system. In the system, the master station device periodically notifies the slave station device of synchronization signals, and also notifies the slave station device of a propagation delay time based on a physical transmission distance to the slave station device. This enables time synchronization with high precision even when a plurality of slave station devices are arranged at different distances.
As a solution to the problem of a fluctuation in the propagation delay, IEEE 1588 also proposes a method in which a transmission device located on the transmission side adds, to a specific field of a timing packet, a link delay that is determined by factors including a retention period of packets in a transmission device, such as a buffer in a layer 2 switch, on a transmission path, a transmission rate of a physical line, such as 10/100/1000 BASE, which constitutes the transmission path, and a packet length. This makes it possible to compensate for a fluctuation in the delay caused due to buffering or line redundancy switching.